1. Field of the Invention
The present invention relates to a fixing device which fixes an unfixed image formed on a recording medium by heat, and which is used in an image forming apparatus such as a copying machine or a printer.
2. Related Background Art
A heating roller type fixing device is widely employed in an image forming apparatus such as a copying machine and a printer. Such a heating roller type fixing device effects fixing of an unfixed image by a heat source such as a halogen heater, while feeding a recording medium supporting the unfixed image by a pair of fixing rollers consisting of a heating roller and a pressure roller to be pressed thereto while pinching the recording medium (see, for example, Japanese Utility Model Publication No. 52-120856).
FIG. 17 shows an example of a fixing device of the heating roller type. A fixing roller 209 has a core bar 209a made of aluminum, iron, or the like which is coated with a mould releasable resin layer 209b such as PFA, PTFE, and the like. The fixing roller 209 further contains a halogen lamp 208 serving as a heat source, and is driven to rotate in a predetermined direction. The surface temperature of the fixing roller 209 is detected by a temperature detecting element 207 which is in contact with the fixing roller 209. The halogen lamp 208 is controlled according to a detection signal from the temperature detecting element 207 so that the surface temperature of the fixing roller 209 is maintained in the longitudinal direction at a constant value suitable for fixing operation.
A pressure roller 210 has a core bar 210a which is made of iron, stainless steel, or the like and which is coated with an elastic layer 210b, wherein the elastic layer 210b is made of a heat resistant and mould releasable material such as silicone rubber, fluoro-rubber, or the like. The pressure roller 210 is pressed against the fixing roller 209, and is driven to rotate with the fixing roller 209 in a predetermined direction.
Thus, a transfer medium P supporting a toner image T on the side of the fixing roller 209 is fed via an entrance guide 217 between the fixing roller 209 and the pressure roller 210, and is fed by them, while pinched at the pressing position therebetween, as the fixing roller 209 and the pressure roller 210 are rotated. During this time, the toner image T on the transfer medium P is heated and pressed by the fixing roller 209 and the pressure roller 210, thereby being fixed on the transfer medium P as a permanent image. The transfer medium P is separated from the fixing roller 209 by a separation claw 218.
In the heat fixing device of the heating roller type, when in low temperature environment, since the pressure roller 210 and the transfer medium P are cooled, heat escapes from the fixing roller 209 to the pressure roller 210 and the transfer medium P. When image output is constantly performed, temperature of the fixing roller 209 during image output is adjusted to be fixing temperature T.sub.2 at which excellent fixation can be maintained.
It is because the pressure roller 210 is heated up by the fixing roller 209 during forward revolution and sheet feed, thus the surface temperature of the pressure roller 210 goes up as shown in FIG. 18. As a result, during sheet feed after print ON of second sheet, an outflow of heat to the pressure roller 210 decreases, thereby improving the fixing property.
If a sheet is intermittently fed, however, for example, if the image forming operation is resumed several seconds after discharging the transfer medium P out of the apparatus main body and stopping the image forming operation, the following problems occur, which depend on whether the temperature of the fixing roller 209 is controlled at the fixing temperature or not during the time when the transfer medium P passes through nip region formed by the fixing roller 209 and the pressure roller 210 pressed thereto, and then discharged to a discharge unit to stop the apparatus (the time is hereinafter referred as "backward revolution") .
1. If the transfer media P are intermittently and repeatedly fed in low temperature environment, the fixing roller 209 is cooled during the backward revolution as shown in FIG. 9. And, since the halogen heater 208 is turned off, the pressure roller 210 is not heated up as efficiently as during continuous sheet feed. Thus, decrease of surface temperature of the fixing roller 209 and the outflow of heat from the transfer medium P to the pressure roller 210 during sheet feeding becomes greater after print ON of the second sheet, so that fixability is deteriorated. PA1 2. If temperature control operation to maintain the fixing temperature is continued after the backward revolution begins, as the pressure roller 210 is heated up during the backward revolution as shown in FIG. 20 and the outflow of heat from the transfer medium P to the pressure roller 210 is reduced. Therefore, even if the sheets are intermittently fed in low temperature environment, an excellent fixing operation is possible. In high temperature environment, however, temperature rising on the discharge side of the fixing device becomes large. It is because decrease of the outflow of heat from the fixing roller 209 to the transfer medium P causes greater heat radiation into the atmosphere around the fixing roller 209, and accordingly, greater temperature rising. Such a case is illustrated in FIG. 21.
As a result, for example, a thin sheet of transfer medium P loses its toughness after sheet feed and tends to coil around the fixing roller 209, thereby causing a considerable curl after separation, and in the worst case, even wrinkle.
As described above, generally, temperature control of the fixing roller 209 during the backward revolution has never been taken into account, wherein temperature control is continued during the backward revolution for the sake of excellent fixing, somewhat, at the cost of prevention against curl and wrinkle of the sheet caused in high temperature environment. Otherwise, generally, the halogen heater 208 is turned off during the backward revolution for the sake of prevention against the curl and wrinkle of the sheet, somewhat, at the cost of exellent fixing.
Note that the above-mentioned problems become remarkable, depending on the kinds of sheets. For example, in the above case 1, the fixing property of a thin sheet whose base weight is 90 g/m.sup.2 or less is relatively good, while that of a thick sheet such as a post card, an envelope, and the like whose base weight is greater than 130 g/m.sup.2 becomes considerably worse.
In this case, the fixing temperature of the post card or the envelope may be set to be higher than that of other kinds of sheets. But, as the phenomena in said case 1 happens only in a low temperature environment, such a countermeasure causes excessive temperature rising in high temperature environment, which is also problematic.
Also, in the above case 2, the problems which are concerned with the postcard, the envelope, and the like can be avoided, but the curl and the wrinkle of the recording medium still occurs when a thin sheet is fed in high temperature environment.